Biology (Holt)

Mass Extinctions

The fossil record indicates that a sudden change occurred at the end
of the Ordovician period. About 440 million years ago, a large per-
centage of the organisms on Earth suddenly became extinct.
is the death of all members of a species. This was the first
of five major mass extinctions that have occurred on Earth. A
is an episode during which large numbers of
species become extinct.
Another mass extinction of about the same size happened
about 360 million years ago. The third and most devastating of all
mass extinctions occurred at the end of the Permian period,
about 245 million years ago. About 96 percent of all species of
animals living at the time became extinct. About 35 million years
later, a fourth, less devastating mass extinction occurred.
Although the specific causes of these extinctions are unknown,
evidence indicates that worldwide geological and weather
changes were likely factors. The fifth mass extinction will be dis-
cussed in more detail in a later chapter. It occurred 65 million
years ago and brought about the extinction of about two-thirds of
all land species, including most of the dinosaurs.
Some scientists think that another mass extinction is occurring
today. These scientists reason that this new extinction is taking
place because the Earth’s ecosystems, especially tropical rain forests,
are being destroyed by human activity, as shown in Figure 10.The
world has already lost half its tropical rain forests. If the current
rate of destruction continues, from 22 percent to 47 percent of
Earth’s plant species will be lost, along with 2,000 of the world’s
9,000 species of birds and countless insect species. This would be
an astonishing loss of biodiversity.

mass extinction

Extinction

SECTION 2The Evolution of Cellular Life 263

ORDOVICIAN PERIOD SILURIAN PE

Animal diversity abounds; first jawless fishes

500 •••••• 440



First mass

extinction

Figure 10 Rain forests

are being destroyed at an

alarming rate.Although

tropical rain forests cover only

7 percent of the Earth’s land

surface, they contain more

than one-half of all the world’s

animal and plant species.

http://www.scilinks.org

Topic: Extinction

Keyword: HX4078

Section 2 Review

Contrastthe two major groups of prokaryotes.

4A 8B 8C

AnalyzeMargulis’s theory of endosymbiosis,

citing its strengths and weaknesses. 3A

Comparebacteria with eukaryotes. 8B 8C

Summarizehow multicellularity advanced the

evolution of protists. 7B

Critical Thinking Justifythe argument that

today’s organisms would not exist if mass

extinctions had not occurred. 7B

The kingdom that includes

both multicellular and unicellular eukaryotes

is called 8C

APlantae. CEubacteria.

BProtista. DArchaebacteria.

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